Growing & Winemaking
Post-Bottling Winemaking
In Wines & Vines’ closures issue last year, I described how the wine industry is beginning to grapple with the issue of oxygen management, focusing on the ongoing research initiative begun in 2007 by synthetic closure company Nomacorc to study the impact of oxygen in winemaking and post-bottling. In this article, we’ll look at some of the newest results from these studies and discuss their significance.
Nomacorc’s vested interest in sponsoring oxygen research is clear: It is currently offering closures with a range of different oxygen transmission (OTR) levels, and so the company would see itself well placed if winemakers were to start choosing closures offering a specific level of OTR to match their wine styles. Correspondingly, as manufacturers of closures that have a reasonably high level of OTR, if the prevailing sentiment in the wine industry were to become one that closures with very little or no OTR were desirable for all wines, then Nomacorc would have some difficulty.
Nomacorc has partnered with respected wine science institutions across four continents, each of which has been looking at particular grape varieties. In addition, Nomacorc is one of the founding members of a nonprofit consortium titled Oxygen in Wines, or O2W. This international association is comprised of suppliers and service providers to the wine industry. Its stated objective is “the promotion of scientifically based solutions for oxygen management challenges in the wine industry.” Participating members include G3, Lallemand, Perrier Bottling Machines, Inter Rhône and Appe.
One of the keys to this research is the development of a quick, easy, non-invasive method of oxygen measurement. To this end, Nomacorc has worked with German company Presens to develop luminescence technology for measuring oxygen concentrations throughout the winemaking process and even after bottling. Nomacorc is now distributing this technology under the brand name NomaSense, and its oxygen analyzer equipment package consists of a Presens Fibox 3 LCD trace oxygen analyzer, together with a range of reusable sensor spots.
To operate the device, attach a sensor spot inside a bottle, direct a light onto the probe and take a read-out. Alternatively, if the technology is to be used invasively, introduce a dipping probe into a tank or barrel. The great advantage of the sensor spot technology is the capability to follow through time the dissolved oxygen in a bottle of wine without opening the bottle. The reusable Presens oxygen sensor spots are precalibrated and withstand normal winery cleaning practices. They come in two different sensitivities, PSt3 and PSt6, which cover all possible oxygen ranges in the winery.
With this sort of technology, it is possible for wineries to carry out thorough oxygen audits. This is something that is much needed: Very few wineries have any idea how much oxygen they are introducing into their wine through different stages of the winemaking process.
Winery oxygen overwhelms closure OTR
Controlling oxygen in the winery is important if the concept of matching closures to wine type is ever to become a reality. Currently, the differences in oxygen pick-up are frequently so large that they render differences in closure OTR insignificant. The noise in the system from poor oxygen management obliterates the nuances of wine style fine-tuning that closures are capable of.
Nomacorc’s Dr. Stéphane Vidal explains that the concept of oxygen management in wines consists of three phases. First, there is macro-oxygenation, which is the relatively large dose of oxygen that wine experiences during primary fermentation. Then there is micro-oxygenation, lesser exposure to oxygen that occurs during barrel aging or aging in tank with the deliberate introduction of small doses of oxygen. Finally, there is nano-oxygenation, which is the very limited exposure to oxygen that occurs post-bottling through closure OTR. Vidal points out that it makes little sense suddenly to alter this sequence by exposing wine at bottling to a high dose of oxygen (macro-oxygenation again), which is what commonly happens under poorly managed bottling conditions.
The ultimate goal of this research is to provide sufficient data enabling winemakers, with some degree of confidence, to use closures with different OTR levels as a winemaking tool. “An important outcome of the AWRI’s (Australian Wine Research Institute’s) research on wine closures is the recognition that when a wine is bottled under different closures, different wines begin to be created from that point onwards,” state the authors of AWRI’s report about their influential closure trial.
“Other workers have apparently expanded this concept to other bottling variables such as the filling height, the concentration of free SO2 at bottling and the mixture of gases in the headspace of bottles post-filling. The ability to link such variables to wine development post-bottling creates the possibility of reliably predicting, and therefore optimising, wine development in bottle.” The notion is that if we were able to gather enough information about how different wine styles respond to oxygen post-bottling, and couple this with our knowledge of closure OTR, we could then offer winemakers the information they need to match closure type to wine style. In this scenario, closure choice becomes an active winemaking decision.
Study of 16 Grenache wines
Results emerging from the Nomacorc-sponsored studies are beginning to fill the gaps in our knowledge and bring the concept of winemaker intention closer to reality. In particular, there have been some interesting results coming from the laboratory of Dr. Véronique Cheynier at INRA Montp
ellier, France.
Cheynier’s laboratory has been studying the influence of oxygen on the evolution of polyphenolics in red wines made from Grenache. Polyphenolic compounds are important in red wine, and they include anthocyanins (which account for color) and tannins. To cut a very long story short, the anthocyanins and tannins change form throughout the winemaking process, combining with each other and other molecules. Exactly how they do this is very important for red wine quality, and oxygen plays a major role.
For this work, a matrix of 16 different Grenache wines was created, comparing extraction techniques, winemaking processes and closure OTR. In the first instance, two extraction techniques were used: traditional maceration and flash release. Flash release (also known as flash détente) involves using steam to rapidly heat grapes to 95°C (203°F) for six minutes and then cooling them down rapidly in a vacuum, which increases the extraction of polyphenolic compounds from the skins.
After fermentation each wine was further subdivided into two batches, one of which was subjected to micro-oxygenation while the other wasn’t. These four different wines were then each bottled in 375ml (half normal size) bottles, in order to amplify the effects of oxygen post-bottling, and were sealed with synthetic (Nomacorc) closures.
Each of the four wines was subdivided further into four batches with varying OTR conditions, thus creating a matrix of 16 different experimental wines. The first batch was sealed with Nomacorc Light closures and stored in air (21% oxygen); the three remaining batches were sealed with Nomacorc Classic closures and stored in air (21% oxygen) or in stainless steel drums at 4% oxygen or 0% oxygen. All wines were kept at a temperature of 23°C. Calculated OTR rates for these four scenarios were 11.9, 8.0, 1.9 and 0.8 μg O2 per bottle per day, respectively.
| Oxygen as a stylistic tool
—AWRI (Sept.-Oct. 2009). “Post-bottling management of oxygen to reduce off-flavor formation and optimise wine style,” Wine Industry Journal, 25(5) 24, 24-28. |
Using a technique called HPLC (high-performance liquid chromatography) the researchers were able to analyze wines for a wide range of polyphenolic compounds. They also did analysis for free and total sulfur dioxide, and looked at the color of the wines using spectrophotometry. The wines were studied 10 months after bottling to look at the effect of post-bottling oxygen exposure.
The researchers used a technique called principal component analysis (PCA) to compare the color and phenolic composition data from the 16 different wines. PCA is a statistical technique used to find patterns in a mass of data. It pulls out from the data the factors that account for the variance in the variables under study—here the color of the wine and the phenolic composition. This showed that the most important factors in explaining the differences in these wines at 10 months are 1) the OTR, and 2) whether the wine was a flash release or traditional maceration wine. Of 20 color and polyphenolic parameters measured, OTR had a significant effect on 18; extraction technique affected 14, and micro-oxygenation affected 11.
The take-home message? After some time in bottle, closure OTR appears to have more influence on red wine polyphenolics (and hence color, structure and mouthfeel) than standard winemaking practices do. But this is just a chemical analysis, and what really matters is linking the changes detected by chemical analysis to changes in how the wines are perceived by consumers. Hence, Cheynier’s laboratory extended this study to sensory analysis.
In a second study, the same 16 wines were subjected to sensory analysis by a panel of 18 trained judges. The panel selected 12 attributes to describe these samples and then rated the wines blind for each, both at bottling (to assess the winemaking differences) and 10 months after bottling.
They found that OTR significantly affected eight of the 12 assessed attributes (while five attributes were affected significantly by winemaking at bottling, only one of these remained significant 10 months after bottling). Wines stored under high OTR had more color intensity and appeared more orange, had differences in odor (higher in “red fruits,” “caramel;” lower in “vegetable” and “animal”). These significant differences are interesting; what would be even more interesting would be to present a range of wines aged under different OTRs to consumers and gauge their preferences.
AWRI looks at Sauvignon Blanc
Another interesting set of research findings is coming from work conducted by AWRI looking at Sauvignon Blanc. Managing oxygen is especially important for this variety, because it presents particular challenges owing to the fact that closely related volatile sulfur compounds can be both olfactory defects and also desirable aromatics.
There are three important sulfur-containing compounds that contribute to the aroma of Sauvignon Blanc: 3-mercaptohexan-1-ol (3MH), 3-mercaptohexyl acetate (3MHA), and 4-mercapto-4-methyl-2-pentanone (4MMP), contributing aromas of grapefruit, boxwood and passion fruit. These thiols (the name of the chemical class of these compounds) are susceptible to oxidation, and so the ideal closure for Sauvignon Blanc wines would seem to be the lowest OTR closure possible, which is currently the screwcap with a tin/Saranex liner. However, results from AWRI’s studies also have
shown that using such a low OTR closure can increase the risk of reductive aromas from the shifts in sulfur compound chemistry that can occur in the low-redox potential environment of a wine sealed this way.
This creates a paradoxical situation where winemakers who are using screwcaps with tin/Saranex liners play it safe by using pre-bottling copper fining to remove the sulfur compounds responsible for reductive off-flavors. The problem is, these sulfur compounds are very close in reactivity to the good thiols (3MH, 3MHA and 4MMP), so this copper fining can negatively affect varietal aroma.
AWRI has shown that copper additions at bottling can reduce the concentration of 3MH in Sauvignon Blanc wines. Interestingly, this effect was only observed at higher concentrations of SO2 (60 mg/L) and not lower concentrations (30 mg/L). But there was an even more interesting observation: After eight months of storage, even in the wines bottled with 30 mg/L of SO2, a decrease in 3MH, 3MHA and 4MMP was seen in copper-fined samples that in most cases was larger than the decrease associated with the use of higher OTR closures. This indicates that for Sauvignon Blanc, a winemaker is better off using a higher OTR closure than a tin/Saranex screwcap in conjunction with copper fining. The authors point out that because copper is a powerful promoter of oxidation (a catalyst), excessive copper fining can accelerate oxidation reactions, resulting in a higher risk of premature oxidation.
Amorim studies white Bordeaux
Finally, a study from Amorim’s Paulo Lopes has also examined the effect of oxy¬gen dissolved at bottling and transmitted through the closure on the composition and properties of a Bordeaux Sauvignon Blanc. This study involved both chemical and sensory analysis, and followed the wine for two years after bottling.
Using a colorimetric technique involving an oxygen-sensitive dye, Lopes and colleagues compared the OTRs of a range of different closures. Lowest was screwcap with a tin/Saranex liner, and highest was a synthetic cork (Nomacorc classic). Microagglomerate cork (Neutrocork), agglomerate cork, screwcap with a Saranex liner, natural cork and colmated cork were in between these two extremes, in that order. In addition, some wine was bottled in a sealed glass ampule, with no OTR.
The chemical analysis showed that the levels of “good” thiols analyzed for (3MH and 4MMP) were highest in the low OTR closures and lowest in the higher OTR closures. The exception here was the screwcap with the Saranex liner—here the level was lower than might be expected from the OTR, leading the authors to suggest that the Saranex was scalping the thiols. The analysis for sulfide (H2S) showed much higher levels in the ampule and screwcap with tin/Saranex liner, consistent with the observation that very low OTR closures carry with them an enhanced risk of reduction.
In the sensory analysis after 24 months in bottle, the two extremes of OTR were not successful for this wine. The Nomacorc-sealed wines showed evidence of oxidation, which masked the fruity characters. The ampule and screwcap with the tin/Saranex liner showed reduction issues that again masked the fruity characters. The intermediate OTR closures performed best in terms of fruit expression, but the agglomerate-sealed wines were ruined by contamination by TCA (2,4,6-trichloroanisole)—analysis showed these levels to be 1–3 ng/L, which masked the fruity characters of the wine. The authors concluded that “an oxygen-sensitive variety such as Sauvignon Blanc benefits from some low oxygen exposure after bottling, at the levels provided by cork stoppers. These wines retained high enough amounts of varietal thiols to maintain the typical box tree and tropical fruit aroma of Sauvignon Blanc, but at the same time kept the deleterious sulfides at very low levels.”
So the evidence seems to be accumulating that winemaking post-bottling is a valid concept, and that winemakers should be thinking about matching closure OTR with wine style. Further studies of this nature would really help make this a practical reality.
London-based writer Jamie Goode is the publisher of wineanorak.com and specializes in wine science issues. His first book was published in 2005 by Mitchell Beazley. Contact him through Wines & Vines at edit@winesandvines.com.
